HI Glenn I have done extensive research on high voltage boards, and found
that z axis degration is a hard number to get correct, the 50 volt per mill
is for safty conserns  UL stuff and to allow for, as you stated micro voids
between layers, it is also way above the typical 1200 v manufactures state ,
I have seen voltage dialectrics and time.   use 2-3 layers of prepreg
between the voltages and as also stated include a test coupon on the side ,
use 5000 volts as a test voltage and your 250-500 will be relaible. I did
write a technical booklet on high voltage and printed circuits if it would
help you at megadawn.com
email me if you need further information  [log in to unmask]

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